Modern digital high-speed communications systems commonly utilize Orthogonal Frequency Division Multiplexing (OFDM) for robust, high data rate communications over impaired channel conditions. Standard technologies include Home Plug, IEEE 802.11a, IEEE 802.16a, ADSL, Digital Audio Broadcasting, . . . , etc.
OFDM is a frequency division multiplexing scheme that utilizes orthogonal sinusoidal sub-carriers to carry data by modulating the phase and amplitude of each sub-carrier. OFDM's inherent advantage as a bandwidth efficient communications scheme includes its ability to overcome severe channel conditions such as frequency selective fading while being relatively simple to implement.
To optimize the capacity between two OFDM communications devices, the channel conditions for each sub-carrier are typically measured and data bits are assigned to each sub-carrier according to the signal-to-noise ratio (SNR) at the intended receiver. In order for the transmitting node to allocate power to each sub-carrier and to modulate data optimally, the intended receiver will have to define the modulation scheme for each sub-carrier and communicate this to the other device. This optimal modulation scheme has to be exchanged between devices with a decodable, lower data rate modulation scheme before the optimization occurs. This exchange of optimum modulation schemes results in an additional complexity and latency between the two devices and has to be updated whenever channel conditions change significantly (especially for the worse).
Different loading algorithms are used for allocating power to sub-carriers and assigning the number of bits. The SNR is a function of the amount of power allocated to a sub-carrier and the channel. In some cases a maximum power spectral density (PSD) is dictated by regulation or practical considerations. The “water pouring” method for allocating power was introduced by Gallager in 1968 (“Information Theory and Reliable Communication”, page 389) and by Wozencraft in 1965 (“Principles of Communication Engineering”, pp. 285-357) and involves allocating available energy to sub-carriers based on the inverse of the channel frequency response subject to additive white Gaussian noise (AWGN). The water pouring method is known to be difficult to implement in practice.